1. Technical Field
This disclosure relates to a coating material produced by a multilayer simultaneous coating process and a method for producing the same, particularly to a coating material which is useful for a thermosensitive recording material and a method for producing the same.
2. Description of the Related Art
Conventionally, when a thermosensitive recording material is produced, an under layer (for heat insulation, sealing of the web, etc.), a thermosensitive recording layer and a protective layer are applied over a web one by one, by blade coating, wire bar coating, rod bar coating or the like.
Today, however, as often used in producing photographic photosensitive materials and the like such as photographic films, multilayer simultaneous coating based upon a slide curtain coating process is becoming popular, in which coating solutions having different functions are discharged from respective slits and deposited over a sliding surface, then the deposited coating solutions are made to fall freely and hit a continuously running web, and a coating film is thus formed.
However, this slide curtain coating process presents such a problem that in comparison with the conventional process of applying layers one by one, moisture evaporation pores created in a coating surface when dried are large in size, and thus the coating surface is uneven. FIG. 1 shows evaporation pores in a coating surface produced by a multilayer simultaneous coating process using a slide curtain coating apparatus. As shown in FIG. 1, large evaporation pores were observed.
The mechanism of the creation of moisture evaporation pores cannot be specified; nevertheless, as far as the conventional process of applying layers one by one is concerned, moisture evaporates from a surface when dried, a solid content is concentrated from the coating surface side, a dissolved resin on the coating web side also moves to the surface side when the moisture moves from the web side to the surface side, a resin film is thusly formed on the coating surface side; moreover, the evaporation rate is high, so that the time spent in forming a film structure when dried is short, and flocculation of dispersed particles hardly takes place in the coating. Therefore, the coating surface is smoother. Meanwhile, as for a product produced by a multilayer simultaneous coating process, a dispersion solution is used for a deposited layer other than a top layer. Accordingly, when a coating surface is created in the form of a film as it is dried, a dispersion solution layer constituting an under layer is still liquid and is therefore gradually dried. Thus, the drying takes place slowly, and contraction of the film also takes place slowly; therefore, the time spent in forming a film structure when dried is long, flocculation occurs amongst dispersed particles in the coating, and moisture is unevenly present in the dispersion solution layer. Accordingly, it is inferred that the following is possible: at a late stage of the drying, when moisture in the dispersion solution layer evaporates, evaporation pores become large in size at a place where there is a great deal of moisture; meanwhile, empty spaces are created by the evaporation, and adjacent particles move so as to fill the empty spaces, thereby making the coating surface uneven. Consequently, regarding the product produced by a multilayer simultaneous coating process, projections and recesses stemming from evaporation pores are created on the coating top layer surface when dried, thereby worsening the glossiness of a thermosensitive recording material product. Hence, faulty products may be produced by this process, which is problematic.
Meanwhile, there is a method disclosed in which an attempt to further prevent the blurring of printed letters/characters caused by color-developing unevenness at a printed portion is made by setting the center line average roughness (Ra75) of a thermosensitive recording surface at 0.5 μm to 2.0 μm, when a thermosensitive recording material is produced by a curtain coating process (refer to Japanese Patent (JP-B) No. 3579392).
However, as to improvement in the glossiness of a coating, which is an object of the present invention, there is such a problem that a desired effect cannot be obtained by merely adjusting the center line average roughness. In addition, the disclosed literature does not disclose how the center line average roughness (Ra75) of the thermosensitive recording surface is controlled. Moreover, it does not disclose the effects which the center line average roughness (Ra75) has on the glossiness of the thermosensitive recording material.